1. Field of the Invention
In a first aspect thereof, the present invention relates to a method for manufacturing a belt structure of a green tire for vehicles.
This invention also refers to a plant for manufacturing the belt structure, a method and a plant for manufacturing a belt package of a green tire incorporating the above-mentioned belt structure, as well as a method and a plant for manufacturing a crown structure of a green tire including the above-mentioned belt package.
2. Description of the Related Art
A green tire is essentially constituted by a composite structure obtained by assembling together a plurality of semi-finished products adapted to constitute a carcass comprising at least one airproof liner, two bead wires to which the edges of at least one carcass ply are associated after interposition of an elastomeric filling fixed to the radially outer surface of each bead wire, a belt structure arranged as a crown around the torically shaped carcass, a tread wrapped around the belt structure and a pair of axially opposite sidewalls, radially extending between the bead wires and the tread.
An additional belt layer incorporating a plurality of circumferentially oriented reinforcing cords, otherwise known as zero degree cords, may be inserted between the belt structure and the tread. Preferably, the above-mentioned additional layer is formed by spirally winding around the belt structure a ribbon (or tape) of rubber mixture incorporating a certain number of such cords.
In the following description and in the subsequent claims, the term xe2x80x9cbelt stripxe2x80x9d is used to indicate a continuous strip of rubber-coated fabric provided with reinforcing cords parallel to one another.
In the following description and in the subsequent claims, the term xe2x80x9cbelt structurexe2x80x9d is used to indicate a composite structure including at least a couple of portions of belt strips, obtained by cutting to size said continuous belt strips. Such structure is formed by radially superposing said portions of belt strips, so that the reinforcing cords incorporated in said portions are parallel to one another in each portion of belt strip and inclined with respect to the cords of the adjacent portion, generally in a symmetrical manner with reference to the equatorial plane of the tire. The portions of belt strips cut to size and radially superposed so as to form the belt structure are called in this way because they are obtained by cutting a substantially continuous strip of indefinite length named xe2x80x9cbelt stripxe2x80x9d.
In the following description and in the subsequent claims, the term xe2x80x9cbelt packagexe2x80x9d is used to indicate the assembly constituted by a belt structure and by an additional radially outer reinforcing layer incorporating the reinforcing cords oriented circumferentially with respect to the tire. Preferably, said additional layer is manufactured by spirally winding onto said belt structure, in subsequent spires axially placed side by side, a continuous ribbon (tape) of rubber-coated fabric of indefinite length, provided with reinforcing cords made of textile or metallic material, longitudinally arranged with respect to such ribbon.
Finally, in the following description and in the subsequent claims, the term xe2x80x9ccrown structurexe2x80x9d is used to indicate a composite structure constituted by a belt package and by a tread arranged in turn as a crown around the additional layer of reinforcing cords of the belt package.
In the field of tire manufacturing, one of the requirements to be satisfied is notoriously that of imparting to the finished product high quality and long lasting characteristics, achieving at the same time a high production capacity of the manufacturing plant as a whole. In this respect, a critical factor which sharply influences the quality of the finished tire is the quality of the semi-finished products which form the tire structure once they have been assembled together.
The quality of the semi-finished products is in turn linked to the production methods and to the environmental conditions in which the semi-finished products are produced and optionally stored.
According to the prior art, in the manufacturing of the belt package of the green tire, the belt strips and the rubber mixture ribbon (or tape), both produced as a continuous ribbon, are wound around spools to form overlapping spires and are subsequently stored until they have to be used in the production line and assembled on an assembly drum.
A first problem connected to this type of handling of the semi-finished products is essentially linked to possible variations of the adhesion, of the chemical-physical characteristics and/or of the dimensional characteristics of the semi-finished products, which take place both during the winding onto the spool, during the storing period and during the various handling steps which such semi-finished products undergo before being used in the production line.
During the storing period in which they are kept in spools, in fact, a change of the humidity and temperature conditions of the storing environment from those considered to be optimal may lead to an unwanted variation of the rubber stickiness which, in case of a reduction thereof, creates process problems during the tire manufacture and, in case of an increase thereof, makes the semi-finished product unusable, generally because it becomes impossible to remove the semi-finished product from the spool with an ensuing waste of material.
A second problem connected to the methods of handling and storing the above-mentioned semi-finished products consists in the packing of the radially innermost layers of material in the spool, due to the weight of the material itself, which packing generally determines the appearance of undesired deformations and stresses which influence in turn the quality of the final product in a negative manner. In the worst cases, such packing of the layers may cause the rejection of the material which may no longer be used.
Another problem related to the above-mentioned handling methods of the semi-finished products, in particular of the belt structure, of the additional layer incorporating the zero degree cords and of the tread, consists in the need to carry out a series of cumbersome operations of loading and unloading of the spools in the assembling machine, operations which require both a continuous need for labor intervention and periodical stops, all to the detriment of the quality of the final product and of the productivity of the plant used for manufacturing the green tire.
The technical problem underlying the present invention is therefore that of providing a method and a plant for manufacturing a belt structure, a belt package and a crown structure of a green tire which enable the achievement both of the desired high and constant quality of the belt structure, of the belt package, of the crown structure and of the semi-finished products that constitute the same, and of a productivity increase of the global plant.
According to a first aspect of the invention, the above-mentioned technical problem is solved by a method for manufacturing a belt structure of a green tire for vehicles, comprising the steps of:
a) preparing, in a substantially continuous manner, a belt assembly incorporating reinforcing cords substantially parallel to one another and inclined at a predetermined angle with respect to the longitudinal axis of the belt assembly;
b) cutting, in a substantially continuous manner, the belt assembly along a cutting direction substantially parallel to the longitudinal axis thereof, so as to obtain two essentially continuous belt strips extending along two conveying directions substantially parallel to one another;
c) supplying, in a substantially continuous manner, said belt strips to at least one assembly drum;
d) superposing on said at least one assembly drum portions having a predetermined length of each of said belt strips, so as to obtain a belt structure comprising radially superposed portions of belt strips wherein said reinforcing cords are parallel to one another in each portion and inclined in opposite direction with respect to the cords of the adjacent portion.
In the following description and in the subsequent claims, the term xe2x80x9cbelt assemblyxe2x80x9d is used to indicate a semi-finished product essentially consisting of a rubber mixture sheet incorporating reinforcing cords inclined with respect to the longitudinal axis thereof and having a width substantially equal to that of the belt strips placed side by side.
In the following description and in the subsequent claims, the expression xe2x80x9cin a substantially continuous mannerxe2x80x9d is used to indicate the absence of intermediate storages of the semi-finished products between the various manufacturing steps. These storages, as said, may change the chemical-physical and/or structural characteristics thereof in an undesired manner. As an example, the expression xe2x80x9cto provide in a substantially continuous manner a belt assemblyxe2x80x9d is used to indicate that the belt assembly is produced and subsequently processed, i.e. cut, without the introduction of an intermediate storage between its production and its subsequent cutting.
Advantageously, thanks to the formation of the belt strips in a substantially continuous manner and immediately upstream of an assembly drum, the method of the present invention enables not only to improve the product quality and the process productivity, but also to reduce the size of the plant, the material wastes and the storage costs, as well as the material handling and labor costs with respect to the methods of the prior art.
Preferably, the method of the invention comprises the step of preparing, in a substantially continuous manner, a belt assembly by means of the following steps:
e) forming, in a substantially continuous manner, a sheet of rubber-coated fabric incorporating a plurality of reinforcing cords substantially parallel to one another;
f) transporting said sheet of rubber-coated fabric along a predetermined conveying direction;
g) cutting said sheet of rubber-coated fabric along a cutting direction forming a predetermined cutting angle with respect to the conveying direction of the sheet of rubber-coated fabric, so as to obtain a plurality of strips of predetermined prevailing length;
h) rotating each strip of an angle equal to said inclination angle of the reinforcing cords with respect to said direction;
i) butt splicing said rotated strips at an edge of prevailing length thereof, so as to form said belt assembly.
In this way, it is advantageously possible to provide, in a substantially continuous manner and without any labor intervention, the strips intended to be spliced to each other to continuously form the belt assembly, in turn intended to provide the belt strips.
According to a preferred embodiment of the invention, each strip is rotated with respect to the conveying direction of the sheet of rubber-coated fabric of an angle substantially equal to the cutting angle of said fabric.
In the following description and in the subsequent claims, the expression xe2x80x9cprevailing length of a semi-finished product (strips, belt assembly, belt strips, portions of belt strips)xe2x80x9d is used to indicate the dimension of the semi-finished product parallel to the laying direction of the cords incorporated within the semi-finished product.
Advantageously, the desired belt structure, comprising radially superposed portions of the belts strips, may be obtained according to the preferred method illustrated hereinbelow.
Preferably, the method of the invention comprises the formation, upstream of the assembly drum, of two belt strips substantially parallel to one another whose reinforcing cords have the same inclination with respect to the conveying direction of the belt assembly.
The superposing step of the portions of the belt strips obtained from the above-mentioned belt strips onto the assembly drum is carried out by tangentially feeding each portion of belt strip respectively one at the top and one at the bottom of the assembly drum, which is rotated in opposite directions during the winding of each portion of belt strip.
In a preferred embodiment of the invention, the superposing step of the portions of each belt strip on the assembly drum is carried out by means of a pair of assembly drums.
In this way, it is advantageously possible to increase the production rate of the plant for carrying out the method of the invention by reducing the time of each assembling cycle of the belt structure.
It should be observed that the portions of belt strips constituting the belt structure, once they have been superposed on one another on the assembly drum, have a different circumferential development. It follows that one of the belt strips, and precisely the one which is intended to form the radially outer portion of belt strip, is used more than the other one. In order to take account of such different use of the two beltstrips, it is convenient and preferable to discard, as a function of the material in excess detected by suitable sensors, portions of the belt strip intended to form the radially inner portions of belt strip. The amount of discarded material generated by said steps of the method of the invention (steps carried out in a substantially continuous manner and immediately upstream of the assembly drum) is however lower than the amount of discarded material normally generated by the production methods of the prior art and due to the aforementioned packing and mutual adhesion phenomena of the continuous belt strips wound on the storing spool.
Preferably, the portions of belt strips are obtained on respective conveying means for conveying the belt strips and the portions of belt strips, said conveying means being provided along the above-mentioned conveying direction of the belt assembly.
According to a preferred embodiment of the method of the invention, the portions of belt strips are supplied to said at least one assembly drum by means of said conveying means for conveying the belt strips and the belt strips portions.
In order to carry out the above-mentioned method, the present invention provides a plant for manufacturing a belt structure of a green tire for vehicles.
According to a preferred embodiment, the plant of the invention provides a splicing device of the strips, which operates substantially by applying pressure perpendicularly to the surface of the strips to be spliced.
Advantageously, such a device reduces almost to zero the damaging strains exercised in directions substantially parallel to the conveying direction of the strips; said strains may cause undesired deformations of the latter, in particular a localized variation of the density of the reinforcing cords.
In this embodiment, it is preferable to extrude the initial sheet of rubber-coated fabric incorporating the reinforcing cords in such a way as to provide the sheet with a lateral lip constituted by elastomeric material. In the subsequent operative steps of forming the strips starting from the initial sheet of rubber-coated fabric, said lip is positioned at one of the edges of prevailing length of the strips, preferably the trailing edge and defines a seat for receivingxe2x80x94by overlapping engagementxe2x80x94the edge having prevailing length, preferably the leading edge, of the following strip, with reference to the formation direction of the belt assembly by means of mutual splicing of the strips as they are obtained.
In this embodiment, the splicing device comprises a plate of suitable weight and size, movably supported above the conveying means for conveying the strips, which plate promotesxe2x80x94by pressing each stripxe2x80x94the adhesion by overlapping between the leading edge of the strip coming from the cutting device of the strips and the underlying lip associated to the trailing edge of the adjacent strip previously already incorporated in the belt assembly being formed. Moreover, the splicing device enhances the butt union between said edges with reference to an operative condition of the plant at full production rate.
Alternatively, the splicing device may comprise a plurality of counter-rotating upper and lower rolls supported above and, respectively, below the conveying means for conveying the strips, said rolls being movable along a direction parallel to the sides of the strips to be spliced.
Alternatively, the splicing device may be of the comb-type, i.e. including a plurality of jaws adapted to grasp the edges of the adjacent strips in order to carry out the butt splicing thereof at their edges having prevailing length.
Advantageously, the plant and the method of the invention enable the preparation of a belt structure of a green tire in a totally automated manner, limiting as much as possible the labor involved and without any need to store the semi-finished products, which are advantageously produced immediately before being assembled together to form the belt structure.
Advantageously, with the plant and the method of the invention, the risks of quality variations of the belt structure related to possible variations of the chemical-physical characteristics of the semi-finished products and/or related to a variation of temperature and/or humidity of the warehouse or to a long permanence of the semi-finished products on the spools are also reduced to a minimum or substantially eliminated.
In a preferred embodiment, the plant of the invention is provided with an extrusion apparatus comprising an extrusion head adapted to supply in a substantially continuous manner a substantially continuous sheet of rubber-coated fabric incorporating a plurality of reinforcing cords, said plurality of reinforcing cords being supplied by a creel supported upstream of said extrusion head.
Downstream of said extrusion apparatus, the plant according to the invention comprises a first cutting device operating on said sheet of rubber-coated fabric leaving the extrusion apparatus so as to obtain a plurality of strips from this sheet.
The plant of the invention also comprises a transfer device of the strips adapted to move the strips away from the cutting position, at which the strips are obtained from the sheet of rubber-coated fabric, towards a releasing position, at which the strips are parallel to one another and arranged side by side along an edge of prevailing length.
Preferably, said transfer device of the strips includes means for moving away the strips and at least one positioning device for positioning the strips.
Advantageously, thanks to said features, the plant lay-out turns out to be independent from the cutting angle of the strips, so that a reduction in the area occupied by the plant is achieved with respect to the plants of traditional type; furthermore, even the strips pickup turns out to be independent from the cutting operation, thus maximizing the plant flexibility.
An additional advantage achieved by the invention and related to the increase of the production flexibility, consists in the reduction of the time required to shift to a production of belt strips for tires of different size, the operations required being limitedxe2x80x94if it is not required to change the type of cord and/or the type of rubber mixturexe2x80x94only to the variation of the cutting angle of the sheet of rubber-coated fabric and/or to the cutting of strips of a different prevailing length.
Furthermore, with respect to the prior art systems based on the use of continuous belt strips provided and packed on spools, it is not necessary to store and handle the spools as well as the various supporting fabrics used, such as the sheets of release material (for example polyester or polyethylene) coupled to the belt strip before carrying out its winding on the spool.
In a preferred embodiment, the means for moving away the strips comprises conveying means for conveying the strips, for example a conveyor belt or any other means having the same function, having a conveying axis substantially parallel to the cutting direction of the sheet of rubber-coated fabric and being movable between the receiving position of the strip arranged downstream the first cutting device of the sheet of rubber-coated fabric and the pickup position of the strips.
Preferably, the plant of the invention further comprises a catching device for catching the sheet of rubber-coated fabric, adapted to transport the free end of said sheet past the first cutting device and cooperating with said transfer device of the strips.
In a preferred embodiment, the positioning device for positioning the strips, adapted to transport the strips from the pickup position, provided downstream of the cutting device of the sheet of rubber-coated fabric, towards the releasing position, provided upstream of the butt splicing device of the strips, comprises a catching device for catching the strips rotatably mounted about an axis perpendicular to the strip surface in such a way as to rotate each strip of an angle equal to the inclination angle of the reinforcing cords with respect to the conveying direction of the sheet of rubber-coated fabric.
Preferably, the catching device for catching the strips (which is part of the positioning device) is mounted on a supporting frame movably guided to and from the strip to be picked-up and positioned.
Even more preferably, the catching device for catching the strips comprises a plate which operates along a direction parallel to the upper surface of the strip, therefore avoiding the creation of undesired stresses in the material constituting the strip, such as the stresses produced when the strip is handled by grasping the same by its edges with subsequent undesired strains, especially in directions substantially parallel to its conveying direction. Such strains may in fact cause undesired deformations of the strips, in particular a localized variation of the density of the reinforcing cords.
According to a further aspect thereof, the invention provides a method for manufacturing a belt package of a green tire for vehicles.
In an embodiment thereof, the width of the additional layer incorporating the zero degree reinforcing cords is substantially equal to the width of the belt structure, whereas its length is at least equal to the circumferential development of the assembly drum. Preferably such length is equal to twice the circumferential development of the assembly drum.
In a preferred embodiment, the additional layer incorporating the zero degree reinforcing cords is formed by spirally winding around the belt structure at least one relatively narrow ribbon of rubber mixture, subsequently referred to as: tape, incorporating one or more reinforcing cords. Conveniently, this tape is wound in such a way as to obtain spires axially placed side by side circumferentially extending around the belt structure substantially for the entire width of the latter.
For the implementation of the above-mentioned method for manufacturing a belt package of a green tire for vehicles, the invention provides a plant.
Preferably, the plant for manufacturing the belt package is provided with an extrusion apparatus, including an extrusion head, adapted to form in a substantially continuous manner the ribbon (or tape) of rubber mixture incorporating the zero degree reinforcing cords.
According to a further aspect thereof, the invention provides a method for manufacturing a crown structure of a green tire for vehicles.
In a preferred embodiment, the tread is fed in a substantially continuous manner to said assembly drum.
In order to carry out the above-mentioned method for manufacturing the crown structure of a green tire for vehicles, the invention also provides a plant.
According to the invention, such plant comprises an extrusion apparatus for supplying a continuous ribbon of rubber mixture from which the treads required for the formation of the crown structure are produced, as well as conveying means for conveying the continuous ribbon towards an assembly drum, on which the previously formed belt package is supported.
Advantageously, after the application of the tread onto the belt package, means adapted to exercise a suitable pressure onto the tread is used in order to increase the adhesion thereof onto the underlying belt package and to strengthen its assembling. Preferably, the above-mentioned conveying means for conveying the continuous ribbon of rubber mixture is provided with cooling means, for example a set of serpentine tubes, in which cold water circulates.